HSCs are responsible for sustaining hematopoietic homeostasis and regeneration after injury for the entire lifespan of an organism through self-renewal, proliferation, differentiation, and mobilization. The mature cell contingent of adult hematopoietic tissue is continuously replenished in the lifespan of an animal, due to periodical supplies from HSCs that reside permanently in the niche. To maintain blood homeostasis, these primitive cells rely on two critical properties, namely multipotency and self-renewal. Multipotency enables differentiation into multiple lineages, while self-renewal ensures preservation of fate upon cellular division. During self-renewal division, an HSC is permitted to enter the cell cycle, while restrained from engaging in differentiation, apoptosis or senescence pathways.
HSCs are rare cells that have been identified in fetal bone marrow, fetal liver, umbilical cord blood, adult bone marrow, and peripheral blood. HSCs are capable of differentiating into each of myeloerythroid (red blood cells, granulocytes, monocytes), megakaryocyte (platelets) and lymphoid (T-cells, B-cells, and natural killer cell lineages) cells. In addition, HSCs are long-lived and are capable of producing additional stem cells (self-renewal). HSCs initially undergo differentiation and commitment to lineage restricted hematopoietic progenitor cells (HPCs), which can be assayed by their ability to form colonies in semisolid media. HPCs are restricted in their ability to undergo multi-lineage differentiation and have lost the ability to self-renew. HPCs eventually differentiate and mature into each of the functional elements of the blood.
HSC transplantation is the only curative therapeutic modality for a variety of hematological diseases. HSCs are also attractive target cells for delivery of genes and gene products to a recipient after transplantation. However, the potential use of HSCs has been limited due to difficulties encountered with obtaining sufficient cell quantities, particularly for adult recipients and those without a matching donor. Furthermore, transplantation of insufficient HSC quantity results in an increased risk of transplantation failure and risks of transplantation complications due to a delay in donor cell engraftment.
Extensive efforts have been invested to expand HSC populations ex vivo. HSCs have been cultured with various hematopoietic growth factors and cytokines, which usually result in the expansion of HPCs, but not HSCs. Ectopic expression of an HSC regulatory transcription factor (such as HoxB4) has been shown to lead to the expansion of HSCs ex vivo, but the safety concern of cell transformation due to gene transfection limits this technique in clinical practice. HSCs have also been co-cultured with other cell types, such as with endothelial cells, mesenchymal stem cells, or bone marrow stromal cells. While these cultures resulted in maintenance or modest expansion of mouse HSC activity, they also resulted in the exhaustion of human long-term HSCs. Further, HSCs have been cultured with small molecules, such as SR-1, which have the ability to promote HSC expansion, but the effects are relatively weak and require several weeks of culture to show an effect.
Despite such efforts to expand HSC populations, only limited success has been achieved. Compositions and methods of expanding long-term HSCs in a population of cells are needed to further medical research and provide therapeutic treatments for conditions and diseases of the hematopoietic system.